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Refactor mllama image processing code, and add pixtral and qwen2vl

New lines can be an important part of a user's prompt and trimming
it can alter the results. We previously only trimmed prompts with
images but refactoring brought this behavior to all prompts, where
it became more noticable.

The /generate endpoint adds less whitespace and therefore doesn't
need to trim it out - this brings the same behavior to /chat.

Thanks to @gabe-l-hart for spotting the issue!

Fixes #7795

Currently we assume that images take 768 tokens of context size for
the purposes of clipping old messages that exceed the context window.
However, our mllama implementation stores the full image embedding
in a single token. As a result, there is significant waste of context
space.

Ideally, we would handle this more generically and have the
implementation report the number of tokens. However, at the moment
this would just result in a similar set of 'if' conditions in the
runner plus APIs to report it back. So for now, we just keep this
simple.

-Update mllama to take the cross attention state as embeddings in
a batch, more similar to how Llava handles it. This improves
integration with the input cache.
-Pass locations in a prompt for embeddings using tags similar to Llava.
-Abstract interface to vision models so the main runner accesses Clip
and Mllama similarly
Co-authored-by: Michael Yang <mxyng@pm.me>

Co-authored-by: jmorganca <jmorganca@gmail.com>
Co-authored-by: Michael Yang <mxyng@pm.me>
Co-authored-by: Jesse Gross <jesse@ollama.com>

* fix system prompt

* execute template when hitting previous roles

* fix tests

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Co-authored-by: jmorganca <jmorganca@gmail.com>

instead of appending image tags, prepend them - this generally produces better results